Modern brakes are more than powerful enough to lock all 4 wheels up with ease, and have the thermal capability to stop a car without issue even if you're going twice the speed limit. The traction of the tires is the limiting factor, not the power of the brakes.

In an emergency stop situation, just slamming the brakes is the fastest way to stop.

Downshifting and using engine braking doesn't stop you more quickly, it lets you continue more quickly. Less heat goes into the brakes (less fade over time), and the engine is in the correct gear to accelerate out (faster exits), which means you can achieve faster lap times in a race.